Sunil R. Kadam

A green process for efficient lignin (biomass) degradation and hydrogen production via water splitting using nanostructured C, N, S-doped ZnO under solar light

Herein, we have reported the simultaneous water splitting and lignin (biomass) degradation using C, N and S-doped ZnO nanostructured materials. The synthesis of C, N and S-doped ZnO was achieved via calcination of bis-thiourea zinc acetate (BTZA) complex. Calcination of the complex at 500 °C results in the formation of C, N, and S doping in a mixed phase of ZnO/ZnS, whereas calcination at 600 °C gives a single phase of ZnO with N and S-doping, which is confirmed by XRD, XPS and Raman spectroscopy. The band gap of the calcined samples was observed to be in the range of 2.83–3.08 eV. Simultaneous lignin (waste of paper and pulp mills) degradation and hydrogen (H2) production via water splitting under solar light has been investigated, which is hitherto unattempted. The highest degradation of lignin was observed with the sample calcined at 500 °C, i.e., C, N, S-doped ZnO/ZnS when compared to the sample calcined at 600 °C, i.e., N and S doped ZnO. The degradation of lignin confers the formation of a useful fine chemical as a by-product, i.e., 1-phenyl-3-buten-1-ol. However, excellent H2 production, i.e., 580, 584 and 643 μmol h−1 per 0.1 g, was obtained for the sample calcined at 500, 550 and 600 °C, respectively. The photocatalytic activity obtained is considerably higher as compared to earlier reported visible light active oxide and sulfide photocatalysts. The reusability study shows a good stability of the photocatalyst. The prima facie observations show that lignin degradation and water splitting is possible with the same multifunctional photocatalyst without any scarifying agent.

Self-assembled hierarchical nanostructures of Bi2WO6 for hydrogen production and dye degradation under solar light

Three dimensional (3D) hierarchical nanostructures of orthorhombic Bi2WO6 with unique morphologies were successfully synthesized by a solvothermal method. The precursor concentration plays a key role in the architecture of the hierarchical nanostructures. A peony flower-like morphology was obtained at higher precursor concentrations, and a red blood cell (RBC)-like morphology with average diameter of 1.5 μm was obtained at lower concentrations. These hierarchical nanostructures were assembled by self-alignment of 20 nm nanoplates. As their band gap is in the visible region, the photocatalytic activity of the Bi2WO6 hierarchical nanostructures for the production of hydrogen from glycerol, and the degradation of rhodamine B (RhB) and methylene blue (MB) under ambient conditions in the presence of solar light was investigated. The Bi2WO6 with peony flower morphology was observed to be the most efficient photocatalyst (H2: 7.40 mmol h−1 g−1, kRhB: 0.240 and kMB: 0.100) of the reported nanostructures. The higher activity of the peony flowers was due to their porous nature, high surface area and lower band gap. Such unique 3D nanostructures of Bi2WO6 have been fabricated for the first time, and their use as photocatalysts in the production of hydrogen from glycerol has hitherto not been attempted. These nanostructures may have potential in ferroelectric, piezoelectric, pyroelectric and nonlinear dielectric applications.

Nanostructured 2D MoS2 honeycomb and hierarchical 3D CdMoS4 marigold nanoflowers for hydrogen production under solar light

Unique two dimensional (2D) honeycomb layered MoS2 nanostructures and hierarchical 3D marigold nanoflowers of CdMoS4 were designed using a template free and facile solvothermal method. The MoS2 structure is depicted with a sheet like morphology with lateral dimensions of 5–10 μm and a thickness of ∼200 nm and a honeycomb nanostructure architecture produced via the self-assembling of vertically grown thin hexagonal nanosheets with a thickness of 2–3 nm. The 3D CdMoS4 marigold nanoflower architecture comprised thin nanopetals with lateral dimensions of 1–2 μm and a thickness of a few nm. The CdMoS4 and MoS2 structures displayed hydrogen (H2) production rates of 25 445 and 12 555 μmol h−1 g−1, respectively. The apparent quantum yields of hydrogen production were observed to be 35.34% and 17.18% for CdMoS4 and MoS2, respectively. The 3D nanostructured marigold flowers of CdMoS4 and honeycomb like 2D nanostructure of MoS2 were responsible for higher photocatalytic activity due to inhibition of the charge carrier recombination. The prima facie observation of H2 production showed that the ternary semiconductor confers enhanced photocatalytic activity for H2 generation due to its unique structure. Such structures can be designed and implemented in other transition metal dichalcogenide based ternary materials for enhanced photocatalytic and other applications.

Effect of zinc:cobalt composition in ZnCo2O4 spinels for highly selective liquefied petroleum gas sensing at low and high temperatures

Nano ZnCo2O4 mixed phase materials were synthesized at varying zinc and cobalt ratios such as 1 : 1, 1 : 1.5, 1 : 2, 1 : 2.5 and 1 : 3. With a change in composition from 1 : 1 to 1 : 2.5, the gas sensing response characteristics increased two times (from 40 to 77.5 kΩ), while at higher zinc cobalt composition (1 : 3) a saturation point is shown (about 80 kΩ). This shows that optimal cobalt loading (1 : 2.5) leads to a two times enhancement in redox ability (from 174 to 346 mmol) and number of active sites, and this upshot significantly helps the sensing response reach a much lower 10 ppm and increases the saturation point to a higher 60 ppm LPG concentration. Furthermore, nano ZnCo2O4 (with a zinc and cobalt ratio of 1 : 2.5) material exhibited an excellent response time (∼80–90 s), rapid recovery time (∼65–75 s), excellent repeatability (fourth cycle), good selectivity (for LPG), higher gas response (∼77.5 kΩ), lower as well as higher operating temperature (from 30 to 250 °C). The results clearly reveal that by tuning cobalt composition in ZnxCo2−xO4, we can achieve maximum sensing efficiency and repeatability.

A stable Bi2S3 quantum dot–glass nanosystem: size tuneable photocatalytic hydrogen production under solar light

The present work comprises a novel approach to design a bismuth sulfide (Bi2S3) quantum dot (QD) glass nanocomposite system by confining nano-Bi2S3 in a designated glass composition for solar light driven hydrogen (H2) production. Numerous methods have been reported for the synthesis of Bi2S3, however, we have demonstrated the synthesis of Bi2S3 QDs (0.5–0.7%) in silicate glass using the melt and quench method. X-ray diffraction and electron diffraction patterns of the glass nanosystem exhibit an orthorhombic crystallite system of the Bi2S3 QDs. Transmission electron microscopy demonstrates that 3–5 and 7–10 nm size Bi2S3 QDs are distributed homogeneously in a monodispersed form in the glass domain and on the surface with a “partially embedded exposure” configuration. The role of glass on the control of the size and shape of Bi2S3 QDs and their effect on the photocatalytic hydrogen generation has been discussed. The maximum H2 production i.e. 6418.8 μmol h−1 g−1 was achieved for the Bi2S3–glass nanosystem under solar light irradiation. This glass nanosystem shows an excellent photostability against photocorrosion and also has a facile catalytic function. Therefore, even a very small amount of Bi2S3 QDs is able to photodecompose H2S and produce hydrogen under visible light. The salient features of this QD glass nanosystem are reusability after simple washing, enhanced stability and remarkable catalytic activity.

Enhanced hydrogen production under a visible light source and dye degradation under natural sunlight using nanostructured doped zinc orthotitanates

The nanostructured Ag and Co doped zinc orthotitanates (ZOT) were synthesized using a combustion method. The structural and optical analysis shows the existence of cubic and tetragonal phases. Morphological study by FESEM reveals the formation of a web like structure along with pot holes by the self-assembly of spherical nanoparticles of ∼50 nm size. Further, TEM investigations reveal diffused and uneven shaped nanoparticles in the range of 10–25 nm. BET surface area measurements show a decrease in surface area due to doping. These ZOTs were employed for photocatalytic dye degradation (Acid Orange-8 and Rhodamine-B) under natural sunlight. The prima facie observations showed Ag@Zn2TiO4 to be an excellent photocatalyst for dye degradation. The kinetics study shows the order of the reaction to be in the range of 1.1–1.41. The ZOTs synthesized have also been used for photocatalytic hydrogen production from H2S under visible light irradiation. It is noteworthy that utmost H2 production (2784 μmol h−1/100 mg) was observed for Ag@Zn2TiO4 which is much higher than that achieved with visible light active photocatalysts reported so far. The dye degradation and hydrogen production from H2S using ZOT are hitherto unattempted. The nanostructured Zn2TiO4 will be a potential visible light active photocatalyst for waste degradation and water splitting.

Nanostructured CdS sensitized CdWO4 nanorods for hydrogen generation from hydrogen sulfide and dye degradation under sunlight.

In this report, CdS nanoparticles have been grown on the surface of CdWO4 nanorods via an in-situ approach and their high photocatalytic ability toward dye degradation and H2 evolution from H2S splitting under visible light has been demonstrated. The structural and optical properties as well as morphologies with varying amount of CdS to form CdS@CdWO4 have been investigated. Elemental mapping and high resolution transmission electron microscopy (HRTEM) analysis proved the sensitization of CdWO4 nanorods by CdS nanoparticles. A decrease in the PL emission of CdWO4 was observed with increasing amount of CdS nanoparticles loading possibly due to the formation of trap states. Considering the band gap in visible region, the photocatalytic study has been performed for H2 production from H2S and dye degradation under natural sunlight. The steady evolution of H2 was observed from an aqueous H2S solution even without noble metal. Moreover, the rate of photocatalytic H2 evolution over CdS modified CdWO4 is ca. 5.6 times higher than that of sole CdWO4 under visible light. CdS modified CdWO4 showed a good ability toward the photo-degradation of methylene Blue. The rate of dye degradation over CdS modified CdWO4 is ca. 7.4 times higher than that of pristine CdWO4 under natural sunlight. With increase in amount of CdS nanoparticle loading on CdWO4 nanorods the hydrogen generation was observed to be decreased where as dye degradation rate is increased. Such nano-heterostructures may have potential in other photocatalytic reactions.

Solar light active plasmonic Au@TiO2 nanocomposite with superior photocatalytic performance for H2 production and pollutant degradation

Spherically shaped plasmonic Au nanoparticles (NPs) of size 10 nm (±4 nm) have been decorated on TiO2 NPs for the synthesis of Au@TiO2 composites via an aqueous sol–gel method. The effects of the Au concentration on the optical properties, structural surface properties and photocatalytic activity have been investigated. The optical study indicated characteristics of the surface plasmon resonance (SPR) of Au NPs, which greatly red shifted the photo-absorption from UV to visible light. A Tauc plot showed a downward shift in the band gap energy due to the creation of a metal–semiconductor Schottky junction at the nano Au@TiO2 surface. The well crystallized mixed (anatase and rutile) TiO2 phase formation was investigated using X-ray diffraction studies. The decrease in the photoluminescence (PL) intensity with Au loading indicated an increase in the charge carrier separation. The porous nature of the synthesized photocatalyst material was observed in both Field Emission Scanning Electron Microscopy (FESEM) and Field Emission Transmission Electron Microscopy (FETEM). The homogeneous distribution of Au NPs over the entire TiO2 surface was examined using FETEM and was also confirmed by elemental mapping with STEM. The broadening and shifting of the Raman peak from 143.2 cm−1 to 144.7 cm−1 indicated the generation of crystalline defects such as vacancies and interstitial rearrangement, which may act as trapping sites for electrons. BET surface measurements revealed that the surface area increased from 29.19 m2 g−1 for bare TiO2 to 60.05 m2 g−1 for 2% (w/w) Au loading on TiO2. The synthesized porous Au@TiO2 composites exhibited a high photocatalytic activity for the splitting of H2O under natural solar radiation with the H2 generation rate of 399 μmol/0.1 g h−1, which is much higher than 132 μmol/0.1 g h−1 demonstrated by pristine TiO2 NPs. Along with the hydrogen (H2) generation via water splitting, photocatalytic Rhodamine-B degradation and the kinetics of the reaction were investigated in solar light. The rate was observed to be due to a pseudo-first order reaction.

3 D Hierarchical heterostructures of Bi2W1-XMoXO6 with enhanced oxygen evolution reaction from water under natural sunlight

Self-assembled 3D hierarchical Bi2W1−xMoxO6 heterostructures with varying x (x = 0, 0.2, 0.4, 0.6, 0.8 or 1.0) with different morphologies were synthesised via a facile one-pot solvothermal method and their photocatalytic activity towards the oxygen evolution reaction (OER) from water under natural sunlight was tested. The structural properties of Bi2W1−xMoxO6 were studied by the X-ray diffraction (XRD) technique, which showed an orthorhombic Aurivillius layered crystal structure. The microstructural features were examined by FE-SEM and FE-TEM techniques which showed that the morphology of Bi2WO6 varies with substitution of Mo and each morphological structure grows via the assembly of tiny nanoparticles of size 50 nm. The effective substitution of Mo in Bi2WO6 extends the optical absorption towards the visible region. The substitution of Mo in place of W was confirmed by X-ray photoelectron spectroscopy. The photocatalytic activities were evaluated by OER under solar light irradiation. The sample Bi2W0.6Mo0.4O6 (S3) shows enhanced photocatalytic activity for OER from aqueous AgNO3 solution (652 μmol h−1 g−1) which is higher than for pristine Bi2MoO6 or Bi2WO6 photocatalysts. Enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region, which enhances the photocatalytic efficiency of the photocatalysts. More significantly, the 3D intrinsically layered nanosheet structure based morphology, and the unique band structure are beneficial for efficient charge transfer, which enhances the photocatalytic activity. This work demonstrates an effective strategy for developing an active photocatalyst with greater utilization of solar light.